The invention relates to an optical multiplexer and demultiplexer, comprising at least one first optical fibre for transporting several light signals of different wavelengths, a wavelength-selective member, further optical fibres for transporting at leeast one light signal in a given waveband, and a lines system which is arranged between the wavelength-selective member and the fibres, the arrangement being such that low-loss transmission paths which are dependent on the wavelength of the light extend between the frist optical fibre and the further optical fibres.
Optical multiplexers and demultiplexers of this kind are known from a publication in "Applied Optics", volume 16, No. 8, Aug. 1977, pages 2180-2194. It has been found that the multiplexers and demultiplexers disclosed in said publication introduce noise into signals transmitted in the form of light pulses. The resultant lower signal-to-noise ratio, of course, is undesirable, the more so if said devices are used in telecommunication networks in which the signal-to-noise ratio should remain as high as possible if signals are to be transmitted over long distances via the optical fibres with a minimum number of intermediate amplifiers.